sim: Update doc to explain submodules
sim: Update doc to explain submodules

Now that we depend on a submodule for mbed TLS code, update the docs to
explain how to fetch this.  Otherwise, the error is somewhat misleading,
just showing a missing sha256.c file.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot